A detailed understanding of the fibrinogen molecule has been a long- standing goal of cardiovascular research because of the physiological role of this key coagulation factor in the control of blood loss and wound healing, and potential pathologic role of the protein in a variety of common diseases, including atherosclerosis, myocardial infarction, and stroke. The long-term aims of this research program are to dissect and characterize the functional elements of fibrinogen and to define the consequences of selected alterations in fibrinogen on hemostasis, wound healing, and disease. Fibrinogen gene-targeted transgenic mice provide a unique experimental system where the endogenous fibrinogen genes can be freely manipulated in vivo and large (i.e., clottable) quantities of recombinant fibrinogen can be generated for biochemical studies. This study will employ currently available transgenic mice lacking fibrinogen (Aalpha chain) and the cloned and sequenced murine Aalpha chain gene to specifically: 1) explore the function of the carboxy-terminal domains of the Aalpha chain, including a) the "carboxy-terminal extension" (alphaC domain) that constitutes ~2/3 of the Aalpha chain, and b) the newly- discovered (and highly-conserved) alphaE domain ("Extended) that is joined to the alphaC domain in a fraction of plasma fibrinogen by alternative splicing; and 2) test the long-standing hypothesis that fibrin(ogen) deposition is a critical factor in the development of atherosclerotic lesions. Under Specific Aim 1, the biochemistry of plasma fibrinogen, clotting function, risk of spontaneous hemorrhagic events, and wound healing will be characterized in transgenic mice in which the endogenous Aalpha products are replaced with forms of the Aalpha chain: i) lacking the alphaC domain, ii) carrying the normal alpha C domain, but with no fraction carrying the alphaE domain, and iii) uniformly carrying the alphaE domain. One working hypothesis to be tested in this study is that alphaE domain promotes the branching of fibrin polymers; therefore, clots formed from fibrinogen that uniformly carries the alphaE domain may be remarkably branched. Under Specific Aim 2, the impact of wholesale loss of plasma fibrinogen on the development and progression of atherosclerotic plaques will be defined. This work will provide the first direct experiment data regarding a "casual" role of fibrin(ogen) in the progression of atherosclerosis. The working hypothesis is that the absence of fibrinogen will reduce the local migration and deposition of cells contributing to plaque development, including platelets, macrophages, and endothelial cells that are each known to carry distinct fibrin(ogen) receptors. If this proves to be correct, then these studies could provide a new foundation for the development of therapeutic strategies to prevent and treat this common vascular disease.